Many approaches use gate capacitance of a transistor to pump charge in a charge pump. The transistor could be N-type or P-type Metal-Oxide Silicon, i.e., NMOS or PMOS, respectively. The gate capacitance of a single-gate transistor, however, is relatively small. To improve pump capacities, a large number of gates and thus a large die area is required. Further, using the gate capacitance is inefficient because, when the transistor is off, there is no gate capacitance and thus no capacitance for the charge pump. For example, when a voltage applied at the gate of the transistor is less than the threshold voltage of the transistor, the transistor is off, resulting in no capacitance. In other words, the gate capacitance exists and the pump function operates only when the gate voltage is above the threshold voltage. Additionally, the threshold voltage is larger at low temperatures and thus further reduces efficiency and pump ability.
Like reference symbols in the various drawings indicate like elements.